Lightning-protector.



N. TESLA.

LIGHTNING PROTECTOR.

APPLICATION FILED MAY 6.1916. 1,266,175. Patented Ma'y14,1918.

W I?? @Mm UNITED sTATEs PATENT OEE-ICE.

NIKOLA TESLA, 0F NEW YORK, N. Y'.

LIGHTNIN (1f-PROTECTOR.

To all whom, t may concern.'

Be it known that I, NIKOLAA TESLA, a citiat New York, in the county and State of ew York, have invented certain new and useful Im- Aprovements in Lightning-Protectors, of

which the following is a full, clear, and eX- act description.

The Iobject of the present invention is vprovide lightning protectors of-a novel and improved design ,strictly in conformity With the true character of the phenomena,'more efficient inaction, and far more dependable in safe-guarding life and property, than those heretofore employed.

To an understanding of the nature of my invention and its basic distinction from the lightning rods of common use, it is necessary briefly to explain the principles upon which my protector is designed as contrast-l ed with those underlying the now-prevailing type of lightning rod.

Since the introduction of the lightning rod by Benjamin Franklin in the latter part of the eighteenth century, its adoption as a means of protection against destructive atmospheric discharges has been practically universal. Its eiicacy, to a certain degree, has been unquestionably establishe'd throu h statistical records but there is genera prevalent, nevertheless, a singular theoretical fallacy as to its operation, and. its construction is radically defective in one 4fea-v ture, namely its typical pointed terminal. In my lightning protector I avoid points, and vuse an entirely different type of terminal.

^ According to the prevailing o inion, the lvirtue of the Franklin type of lig tning rod is largely based on the property of points or sharp edges to give off electrlcity into the air. As shown by Coulomb, the quantityof electricity per unit area, deslgnated by 1m electrical density increases as the radius of curvature of the surface is reduced. Subsequently it was proved, by mathematical analysis, that the accumulated charge created an outward normal-force equal yto 21: limes thc square-of the density, and experi'- nieut has demonstrated that when the latter exceeds approximately 20 C. G. S. units, a streamer or corona 'is formed.A From these observations and deductions it is obvious that such may happen at a comparatively low pressure if the conductor is of extremely Speciiication of Letters liatent. ]?a,tentd May 14, 1918. Application led May 6, 19.16.' Serial No. 95,830. I

small radius,lor pointed, and it is pursuant to a misapplication of these, and other,

.truths that the commercial lightning rod of daries arranged on surfaces of large radii of curvature on two dimensions.A The principles which underlie my invention and correct application of which dictate the form and manner of installation of my protector, I Will now explain in contrast Withthe conventional pointed lightning rod.

In permitting leakage into the air, the needle-shaped lightning-rod is popularly believed to perform two functions: one to drain the ground of its negative electricity, the other to neutralize the positive of the clouds. To some degree, it does both. But a systematic study of electrical disturbances in the earth has made it palpably evident that the action of Franklins conductor, as commonly interpreted, is chiefly illusionary. Actual measurement proves the quantity of electricity escaping even from many points., to be entirely insignificant when compared with that induced within a considerable terrestrial area, and of no moment whatever in thel process of dissipation. But it is true that the negatively 'charged air in the vicinity of the ro rendered conductive.

through the infiuence of the same, facilitates the passage of the bolt. Therefore it increases the probability of a lighting. dischargel in its vicinity. The fundamental facts underlying this type of lightning-rod are: First, it. attracts li htning, so that it will be struck oftener t an would be the building if it-were not present; second, it renders harmless most, but not all, of the discharges lwhich it receives; third, by rendering the air conductive, and for other reasons. it is sometimes the cause of damage toneighboring objects; and fourth, on t e whole, its power of preventin injury red'ominates, more or less, over t e hazar s it invites.

My protector, by contrast, is founded on principles diametrically opposite. Its terminal has a large surface. It secures a very low density and preserves the insulating qualities of the ambient medium, thereby minimizing leakage, and in thus acting as a quasi-repellant t0 increase enormously the safety factor. l

For the bestand most economical installation, of protective devices according to 'my invention, those factors and phenomena that these depend, partially, as I shall show,

upon the character of the landscape proximate'to the building site. l'

. In the drawings,y Figures 1 to 4: inclusive, are diagrams requisite to illustration of the facts and conditionsy relevant to the deterf mination of s eciic installations of my iiivention, and igs. 5 to. 8v illustrate construcv tion and application of the protectors. Specifically: v

Fig. 1 is a landscape suited for purpose of explanation; Figs. 2, 3 andiare theoretical diagrams; Figs. 5 and 6 illustrate forms of improved protectors; and Figs. 7 and 8 show buildings equipped with the same.

In Fig. 1, 1 representsLord Kelvins reduced areay of the region, which is virtually part of the extended unruiled ocean-surface. (See Papers on Electrostaties and M aguetism by Sir William Thomson). Under 0rdinary weather conditions, when the sky is clear, the total amount of ,electricity distributed over the land is nearly the same as that which would be containedl within its horizontal. projection. But in times of storm, owing to the inductive action of the clouds, an immensev char e may be accumulated in the locality, t e density being vnegative sense, is m2.

mveau are exposed to ever so much more greatest at the' most elevated ort'ions of the' ground. Assumingthis, un er the'conditions existing at any moment, -let another spherical surface 2, ,coimentric with the earth, be drawn--which maybe calleduelec-` tricalV niveau-such .that thev quantities ,stored over and under it are equal. In othery words, their algebraic sum, taken relatively to the imaginary surface,'in-the positive and- Obects above the risk than'those belowJThus, a building atvy 3,'on a site of excessive density, is apt tabel hit sooner or later, while one in a depression 4, where the charge per unit area is very small, is almost entirely safe. It "follows lthat `the one' building 3 requires more ex tensive equipment than does the other. In

instances, however, the probabilitygof.

their niisapplication due to the want of lfuller appreciation has doubtless been responsible for the Frankliniightning rod taking its conventional pointed form, but

theoretical considerations, and the imp ortant discoveries that have been made in the.

course of investigations with a wireless transmitter of great activity by which arcs of a volume and tension comparable to those occurring in nature were obtained (Prob` lems of Increasing Human Energy Century M agasz'ne June 1900 and Patents 645,576,

'649,621, 787 ,.412 and 1,119,732) at once establish the fallacy of the hitherto prevailing notion on which the Franklin typeof rofl is based, show the distinctive novelty of my lightning protector, and guide the constructor in the use of my invention.

In Fig. 2, 5 is a small sphere in contact with a large one, 6, partly shown. Itcan be proved by the theory of lelectric images that when the two bodies are charged the mean density on the small one will be only Maxwell); In Fig. 3, the two spheres 7 and 8 are placed some distance apart and connected through a thin wire 9. This system having been excited asbefore, the density onv the'small sphere is likely to be many times that on the large-one. Since both are at the same potential it follows directly .that the densities on them will be inversely as their radii of curvature. If the density of 7 b e designated as d and the radius 1', then the charge g=41w2d, the potential prima and the outward force, normal to the surface, )hi-2MP. As before stated, when (l surpasses 20 C. G. S. units, the force f becomes su'iiciently intense to break down the dielectric anda streamer'or corona appears.

In this case 0:80am Hence, with a sphere A of one centimeter radius disruption would .take place at a potentialnp=801cl=251-328 E. S. units, or 75,398.4 volts. In reality,`thedischargeV occurs at a lower pressure .as a consequence of uneven distribution on the small sphere, the density being greatest on the side turned away from the large one.

times greater than that on the other, (See Electricity ami M agnetsnt by Clerk Y In this respect the behavior of 'a pointed conductor is just the reverse. Theoretically. it mht erroneously be inferred from the prec ing, that sharp projections would permitl electricity lto escapeat the lowest potentials, butthis does not follow. The reason willbeclea-r from an inspectionfof Fig.

meant 4, in which such a needle-shapedconductor l10, is illustrated, a minute portion of its tapering end being 'marked 11;* Were this portion removed from the large part 10 and electrically connected with the same through an infinitely thin wire, the charge would be given off readily. vBut the resence of 10 has the eifect of reducingt e capacity of 11, so that a much lhigher pressure 4is required to raise Vthe density to-.the critical value. The larger the body, th e morepronounced is this influence, which is also dependent on configuration, andis maximum for a sphere. When the same is of considerable size it'takes a much greater electromotive force than under ordinary circumstances to produce streamers from the point. To explain this apparent anomaly attention is called to Fig. 3. 'If the radii of the two spheres, 7 and 8, be designated r and Brespectively, their charges g and 'Q `and the distance between their centers D, the potential at 7, due to Q is `But-77 owing to the metallic' connection 9, is at the pot'el'itial-y When D is comparable to R, the medium surrounding the small sphere will ordinarily AVbe at a potential not much different from that of the latter and millions of volts may have to be applied before streamers' issue, even from sharpprotruding edges. yIt is important to bear this in mind, for the earth is but a vast conducting globe. It

i follows that a pointed lightning-rod must posed emissive effect, is in part neutralized by the increasing size below the eXtreme end,

be run far above ground in order to operate at all, and from the foregoing it will be apparent that the pointing of the end, for supand the larger the rod, for reduction of electrode resistance, the more pronounced is this counter-iniuence. For these reasons it is important to .bear in mind that sulicient thickness of the'rodfor very low electroderesistance is rather incompatible with the high emissive capability sought in the needle-like Franklinrod, but, as hereinafter set forth, it is wholly desirable in the use ofmy invention, wherein the terminal con' struction is intended for suppression of charge-emission rather than to foster it.

The notion that Franklins device would be effective in dissipating terrestrial charges 'may be traced to early insulated electriied body. But the inappliexperiments with static frictional machines, when a needle was found capable of q-uickly draining an cability of this fact to th'e conditions of lightning protection will be evident from examination of the simple theoretical principles involved, which at the same time sub.

stantiate the desirability of establishing protection by avoiding such drainage. 'The density at t e pointe end f should be in- :versely as the radiusof curvature of the surface, fbut such acondition is unrealizable.

Suppose Fig. 4 to represent a conductor of radiusV 100.times lthat of the needle; then, although its' surface per unit length is greater 1in the. same radio, the capacity isv onlydoubl'e. Thus, while twice the quantity of electricity is stored, the density on/ the rod is but one-fiftieth of that on the need'1e, from whichit follows that the' latter is far more eicient. But the emissive power of any v such conductor is v circumscribed. Imagine that the pointed (in reality blunt or rounded) endbe continuously reduced in size so yas to approximate the ideal more and more. Duringthe process of reduction, the

den/sity willbe increasing as the radius of curvature gets smaller, but in a proportion distinctly ess than linear;` on the other hand, the area ofthe extreme end,l that is, v the section through which the( charge passes 'out into the air, will be diminishing as the square of the radius. This relation alone imposes a definite limit to the performance of a pointed conductor, and it should be notice that the electrode-resistance would be augmented at 'the same time. Further- -m0re, the eiicacy of the rod is much vimpaired through potential due to the charge of the ground, as has been indicated with reference to Fig. 3. Practical estimates of the electrical uantities concerned in natural disturbances s ow, moreover, how absolutely impossible are the functions attributed to' the ointed lightning conductor. A single clou' ma contain 2 1012 C. G. S. units, or more, in ucing in the earth an equivalent amount, which a number of lightning rods' could vnot neutralize in many years. Particularly to instance conditions that may Ihave to be met, reference is madeto the Electrical World of March 5, 1904, wherein it ap ears that upon one .occasion approximate y 12,000 ,strokes occurred within two hours within a radius oflvless than 50 kilometers from the pla/ce of observation.

But although the pointed lightning-rod is' quite ineletivein the one respect noted,

it has the property of attracting lightningpresented 'by the vast :forces of naturev great Furthermore, la correctitpplication ofthe truths that have thus been explained with l reference to the familiar pointed type of lightning-rod not only substantiates the theoretical propriety of the form 'in which I develo my improvedlightning protector, but wiil lead the` installing engineer properly to take cognizance of those conditions due to location of the building, with respect to surrounding earth `formati/ons and other buildings, probabilities of maximum potential-differences and charge-densities to be expected under the prevailing atmospheric conditions of the site, and deslrable electrode resistance and capacities of the protectors installed.

The improved protector, as above stated,

' behaves in ama-nner just opposite to the Franklin type and is incomparably safer for this reason. The result is secured by the use4 of aterminal or conducting surface of large radius of curvature and suflicient area to make the density very small and thereby A prevent the leakage of the charge and the ionization of the air. The device may be greatly varied in size and shape but it is essential that all its outer conducting elements should be disposed along an ideal enveloping surface of large radius and that they should have a considerable total area.

In Fig. 5, Fig. 6, Fig. 7 and Fig. 8, different vkinds of such terminals and arrangements offsarne are illustrated. In Fig. 5, 12 is a cast or spun metal shell of ellipsoidal outlines, having on its underV side a sleeve with a bushing 13 of orcelain or other insulating material, a apted to be -sl1ppe d tightly on a rod 14, which may be an ordanary lightning conductor. Fig. 6 shows a terminal 15 made up. of rounded or fiat metal bars radiatin from a central hub,

and in electrical contact with the same. The special object of this type is to reduce the wind resistance, but it is essential that the bars have a sufficient area to insure small density, and also that they are( close enough to make the aggregate capacity nearly equal to that of a continuous shell of the same outside dimensions. In Fig. 7 a cupola-shaped and earthed roof is carried by a chimney, serving in this way the twofold practical purpose of hood and protector. Anykind of metal may be used in its construction but it is indispensable that its outer surface should be free of sharp edges and projections irom which streamers might emanate. In like manner lnuiiiers, funnels and vents may be transformed into effective lightning protectors if equipped with suitable devices or designed in conformity with this invention.' Still another modification is illustrated in Fig. 8 in which,-instead of one, four vgrounded barsare provided with as many s un, shells or attachments 18, with the obvious obj ect of reducin the risk.

From the foregoing 1t will be clear that in I dent is rendered extremely improbable owv ing to the fact that there are everywhere polnts and projections on which the terrestrial charge attains a high density and where the air is ionized.y Thus the action of the im-V proved protector is equivalent to a repellant force. This being so, it is not necessary to supportit at a great height, bu the ground connection should be made wit the usual care and the conductor leading to it must be of as small a self-induction and resistance as practicable. l

I claim as my invention:

1. A lightning protector consisting of an elevated terminal, having its outer conduct-- in boundaries arranged on surfaces of large raii of curvature in both dimensions, and a grounded conductor of small self-induction,

as set forth.

2. A lightnin protector composed of a metallic shell o large radius of curvature, and a grounded fconductor of small self-induction, as described.f

3. Apparatus for protection against atmospherlc discharges comprising an earth connection of small resistance, a conductor of small self-induction and a terminal carried by the same and having a large radius of curvature in two dimensions as. and for the purpose set forth. 4. In apparatus for prgtection against atmospherie discharges an insulated metallic which is supported lrectly on a similar rod shell of large radius of curvature supported b a grounded conductor and separated from t e same through a small air-gap as, and for the purpose described. y A

5.A` lightning protector comprising, in. combination, an elevated terminal of large area and radius of curvature in two dimensions, and a grounded conductor of small self-induction, as set forth. j

6. In ap aratus for protection against lightning discharges, the combination of an elevated metallic roof of large area and radius of curvature in two dimensions, and a grounded conductory of small .self-induction and resistance, as described.

7 As an article of manufacture a metallic shell of large radius of curvature provided with apsleeve adapted for attachment to a lightning rod as, and for the purpose set forth.

8. A lightning protector comprising an ellipsoidal metallic shell and a grounded 5 conductor of small self-induction, as set forth. y

9. In apparatus for protection against at# mospherc discharges a cupola-shaped metallic terminal of smooth outer surface, in y combination with a grounded conductor of 10 small self-induction and resistance, as described.

In testimony whereof I aiiix my signature.

NIKOLA TESLA. 

